Yanmar YSE Gearbox - what have i got?

[SteveTibbetts]

Just bought a Yanmar YSE12 to replace my non running YSE8. (woohoo 50% power increase!)

New gearbox looks like it has a plate saying YSE12G, i believe this to mean it has a 3:1 reduction.

The box on the 8 has no plate on it.

Anyone know how to tell if existing box is 3:1 or 2:1 ? In gear and turn the engine thru a revolution?

What do i do if they are different? - change the box so i'm using the one off the 8 with the 12 or look at changing the prop?

Boat is a 24ft Snapdragon 747, i'm obviously not interested in setting any speed records


Thanks in advance

Steve

 

[Tranona]

First call EP Barrus who will probably have the records. Failing that turn the engine as you suggest. Clear difference between 2 and 3:1. If it is 3:1 you may have difficulty swinging a big enough diameter prop. When you have the ratio check out the prop size using the Propcalc programme on www.castlemarine.co.uk

 

[SteveTibbetts]

Many thanks for your help with this, that link is really usefull

Steve

 

[SteveTibbetts]

Prop diameter

Looks like i will have a 3:1 gearbox going in to replace a 2:1. (but with the aforementioned increase from 7hp to 10hp). The very useful Castle Marine prop calculator does indeed tell me that my prop will be undersized and theres not enough room for one the right size, it does make a suggestion that the current prop diameter could be used but the boat speed will be restricted.
It also tells me my existing prop is also too small with the 2:1 ratio and will not give a suggestion, whever or not i increase the prop diameter.
I've not had this boat in the water yet so i've no idea how it performs other than the sellers memory of 12 years ago; he tells me the boat moved about as fast as he would expect with a small diesel in it.
My plan was, and still is, to get the boat in the water as soon as possible, at the minimum spend to be safe and sailing.
Anybody care to hazard a guess what changing the ratio from 2:1 to 3:1 will do to boat speed? My thinking is that if the boat was doing 6kts before it will now do 4kts. I could probably live with that for a time until i can find a new prop / gearbox / engine&box sometime in the next year

As always all advice, however contradictory, is welcome

(and if anyone has a 9" LH prop going spare i could borrow...)